1. Field of the Invention
The present invention relates generally to Wien filters, such as those used in charged-particle beam systems and for other purposes.
2. Description of the Background Art
FIG. 1 illustrates the operating principle of the Wien filter. This type of velocity (momentum) filter uses crossed electric and magnetic fields to exert opposing forces on charged particles passing through the filter. An X-Y-Z orthogonal coordinate system 8 is shown.
A substantially homogeneous magnetic field 10 of magnitude B is directed parallel to the Y-axis in the negative direction. A substantially homogeneous electric field 12 of magnitude E is directed parallel to the X-axis, also in the negative direction. A beam of charged particles 14 is directed initially (before encountering the fields 10 and 12) parallel to the Z-axis in the positive direction, and through the electric and magnetic fields. The fields 10 and 12 are positioned in space such that along trajectory 14, the magnitudes of fields 10 and 12 are always in the same proportion, rising from initial values of zero Gauss (G) and zero Volts/centimeter (V/cm) to some well-defined maximum values, then decreasing back to zero G and zero V/cm again.
Fields 10 and 12 apply forces to the beam of particles 14. The first Equation 16 expresses the force on beam 14 due to electric field 12. This electric force is in the −X direction for positive (+) particles as shown by force vector 18 in FIG. 1. The second Equation 20 expresses the force on beam 14 due to magnetic field 10. This magnetic force is proportional to the vector cross-product of the velocity v of each charged particle in beam 14 and the strength, B, of magnetic field 10. In this case, this magnetic force is in the +X direction for positively-charged particles as shown by force vector 22 in FIG. 1.
A conventional Wien filter is configured such that the force vectors 18 and 22 are equal in magnitude. As such, the electric and magnetic forces 18 and 22 will cancel each other for a charged particle traveling in one direction along the z-axis, while the electric and magnetic forces 18 and 22 will add together to a larger force (double the individual forces) for the same charged particle traveling in the opposite direction along the z-axis. This is due to the fact that the direction of the magnetic force 22 depends on the direction of the velocity vector 14 of the particle, while the direction of the electric force 18 is independent of the velocity vector 14. For example, consider the specific case where the charged particles are electrons. The electric and magnetic forces will cancel each other for electrons going in the negative z direction, while the electric and magnetic forces will add together for electrons going in the positive z direction.
FIGS. 2A and 2B show a typical design for the magnetic field pole pieces 40 and 42, and electric field pole pieces 44 and 46, of a conventional Wien filter. In FIG. 2A, the magnetic field lines are shown as they extend through the magnetic material of pole pieces 40 and 42 (both at zero volts) and between them in the space 48, through which beam 14 passes. FIG. 2B shows the electric field lines which result when voltages +V and −V are applied to electric pole pieces 44 and 46, respectively.
As pointed out in the present disclosure, one disadvantageous aspect of a conventional Wien filter is the chromatic aberration that is induced. Charged particles in a beam of different speeds are deflected to different angles by a conventional Wien filter. In other words, the change in trajectory caused by a conventional Wien filter depends on the energy of the charged particle.
It is desirable to improve charged-particle beam apparatus. It is also desirable to improve Wien filters. In particular, it is desirable to reduce the chromatic aberration caused by Wien filters.